Integral stack columns

ABSTRACT

Systems and methods for power conversion are illustrated. Power conversion architecture for fuel cell systems in particular are described that use dual bus architectures having stack segment pairs and a center-tapped neutral line, and/or an architecture employing integer multiple of three DC/DC converter branches.

The present application is a continuation of U.S. application Ser. No.11/797,708, filed May 7, 2007, the entire contents of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of power conversionsystems. More particularly, embodiments of the present invention employvarious architectures of power conversion systems and various methods toconvert power. Still more particularly, embodiments of the presentinvention relate to power conversion systems for power generated by fuelcells.

Fuel cell systems are currently being used in a variety of power supplyapplications. If an application requires a particular voltage orcurrent, fuel cells are combined into units called “stacks” in which thefuel cells are electrically connected in series to meet therequirements. A generalization of a fuel cell stack is the so-calledfuel cell segment or column, which can contain one or more fuel cellstacks. In certain applications, many fuel cell segments may be requiredfor higher power due to the limitation of current carrying capability ofthe fuel cells. For reliable operation, individual stack current can becontrolled, to improve fuel utilization.

Fuel cells generate power that is converted in a fuel cell powerconversion system, also known as a power conditioning system. A powerconversion system is a system that alters the characteristics of powerproduced by a source in some way. For the case of fuel cells, whichgenerate DC (direct current) power, this can mean the conversion of theDC power to different voltage and/or current levels, the conversion toAC (alternating current) power with a particular RMS (root mean squared)voltage, the generation of three-phase AC power, or all of the above.Typically, a change in the voltage level of a DC source can beaccomplished using a DC/DC (direct current/direct current) converter,whereas the change from DC to AC is accomplished using a DC/AC (directcurrent/alternating current) converter or inverter.

The expected increase in fuel cell use in the future, in terms of bothvolume and number of applications, requires that the design andconstruction of fuel cell power systems be made as efficient aspossible. To facilitate the design and manufacturing of fuel cell powercontrol systems, it is advantageous to allow design flexibility whileminimizing the number of components required to produce such anarchitecture.

SUMMARY OF THE INVENTION

As discussed hereinafter, one embodiment of the invention employs apower conversion system that is constructed with a two-bus approach andcenter-tapped neutral line. Such an embodiment is used to facilitate amodular approach and control of power drawn from the fuel cell stack bythe power conversion system. Furthermore, it is advantageous in someembodiments to have a fuel cell power conversion system where the numberof individually wired stack columns in a fuel cell hot box is evenlydivisible by 3 and 2 (meaning evenly divisible by 6) in order to achievean optimum power electronics architecture.

One exemplary embodiment relates to a fuel cell circuit, comprising: afirst fuel cell segment having positive and negative terminals; a secondfuel cell segment having positive and negative terminals; a neutralline; and wherein the negative terminal of the first fuel cell segmentis electrically connected to both the positive terminal of the secondfuel cell segment and the neutral line.

Another embodiment relates to a power conversion module, comprising: aplurality of DC/DC converter branches, each branch comprising a DC/DCconverter; and a plurality of series connections comprising two fuelcell segments; wherein each DC/DC converter branch is connected to atmost one of the series connections comprising two fuel cell segments;and wherein the total number of DC/DC converter branches is an integermultiple of three.

Yet another embodiment relates to a method for converting DC to AC,comprising accepting a first output of a first series connectioncomprising two fuel cell segments at an input of a first DC/DCconverter; accepting a second output of a first series connectioncomprising two fuel cell segments at an input of a second DC/DCconverter; accepting a first output of the first DC/DC converter at afirst input of a first inverter; accepting a first output of the secondDC/DC converter at a second input of the first inverter; and generatinga first AC output from the first and second inputs of the first inverterrelative to a neutral line connected to a reference potential.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block cell circuit diagram illustrating a fuel cell powerconversion system architecture embodiment.

FIG. 2 is a block cell circuit diagram illustrating a second fuel cellpower conversion system architecture embodiment.

FIG. 3 is a block cell circuit diagram illustrating a third fuel cellpower conversion system architecture embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a fuel cell power conversion system architectureembodiment 100 with two parallel busses is shown. The architecture 100is illustrated from fuel cells to three-phase power output, and may bereferred to as a type of power conversion module, where “module” is ageneral term referring to a group of system components. The system 100comprises two fuel cell segments 102 and 104 containing one or more fuelcells and having respective connections 106 and 108 to DC/DC converters110 and 112. DC/DC converters 110 and 112 are similar converters. Theyare usually boost converters. DC/DC converters 110 and 112 haverespective output busses 114 and 116, referred to hereinafter as the +veand −ve busses, respectively. Each circuit branch having a DC/DCconverter and extending, for example, from connection 106 through DC/DCconverter 110 and ending with bus 114 may be referred to as a DC/DCconverter branch. If the fuel cell segments are included, theterminology “stack column” or “segment column” may be used.

The fuel cell segments 102 and 104 are connected in series at node 132,which is also connected to neutral line 122. Node 132 highlights thatpossibility that fuel cells, stacks and segments of the herein describedembodiments can be individually wired, which allows a system designer toadvantageously connected mid-cell, mid-stack and mid-segment potentialsto a system bus. Here, “individually wired” means that the fuel cell orstack end plates which normally comprise the terminals of a fuel cell orstack are not electrically connected by face to face contact with otherend plates, but are rather electrically connected by a conductor, suchas a wire.

The output of DC/DC converter 110 is connected to +ve bus 114, which isconnected over a voltage drop of +ve by capacitor 118 to node 134, whichis connected to and at the same potential as neutral line 122. Theoutput of DC/DC converter 112 is similarly connected to −ve bus 116,which is connected over a voltage drop of −ve by capacitor 120 to node134. Capacitors 118 and 120 also serve to smooth AC components of thesignals carried by +ve bus 114 and −ve bus 116, known as “ripplecurrents”.

The +ve bus 114 and −ve bus 116 serve as inputs to DC/AC converter orinverter 124, which generates three-phase outputs 126, 128 and 130. Thethree-phase outputs have a desired RMS (root mean-squared) potentialover neutral line 122 and 120 degree phase offsets relative to oneanother. In the embodiment 100, all three phases are generated by DC/ACconverter or inverter 124, although this is not strictly necessary.Neutral 122 is already efficiently provided by the dual busarchitecture.

The split fuel cell connection as shown with fuel cell segments 102 and104 provides high efficiency for a power conversion system with its dualbus architecture. The split bus provides a neutral and facilitates easyparalleling of converters. The efficiency of the architecture derivesfrom the fact that two fuel cell segments 102 and 104 are used, whichfacilitates the construction of counterpoised +ve and −ve busses 114 and116 respectively and the easy generation of a neutral line 122 betweenthe two. It will be apparent that the dual bus architecture may bemultiplied and extended to systems involving integral multiples of fuelcell stack systems arranged according to the configuration of FIG. 1.

Referring now to FIG. 2, a second embodiment in the form of a fuel cellpower conversion system architecture 200 is shown. The architecture 200may also be referred to as a power conversion module. The embodiment ofFIG. 2 comprises three pairs of fuel cell segments. Fuel cell segments202 and 204 are paired in an arrangement similar to that described withreference to FIG. 1. Fuel cell segments 206 and 208, as well as fuelcell segments 210 and 212 are also similarly paired. As an example, fuelcell segment 202 is connected with its negative terminal to node 226,while fuel cell segment 204 is connected with its positive terminal tonode 226. Fuel cell segments 206 and 210 are also connected with theirnegative terminals to nodes 228 and 230 respectively. Nodes 228 and 230are connected to nodes 226 and 254 as well as neutral line 232, and areat the same electrical potential. Fuel cell segments 208 and 212 areconnected with their positive terminals to nodes 228 and 230respectively.

Each fuel cell segment 202, 206 and 210 has an output bus 214, 216 or218 respectively. The output busses feed fuel cell segment output to aDC/DC converter 234, 236 or 238, respectively. The DC/DC converters 234,236 or 238 are generally boost converters and serve to bring the +veoutput bus 264 to a voltage level with respect to neutral line 232appropriate for the power system application. The negative terminals offuel cell segments 204, 208 and 212 are also connected to output busses220, 222 and 224 respectively, leading to DC/DC converters 244, 242 and240 respectively. These DC/DC converters serve the same function asDC/DC converters 234, 236 and 238, albeit with an opposite polarity,bringing the voltage on −ve bus 266 to a suitably negative level withrespect to neutral line 232.

The +ve bus 264 is joined at node 246, placing the DC/DC converters in aparallel circuit architecture. A voltage drop occurs at capacitor 250 toneutral line 232 at node 254. Capacitor 252 also serves to smooth outripple currents produced by DC/DC converters 234, 236 and 238.Similarly, −ve bus 266 is joined at node 248, placing DC/DC converters240, 242 and 244 in a parallel circuit architecture. A voltage increaseoccurs from node 248 over capacitor 252 to neutral line 232 at node 254.Capacitor 252 also serves to smooth out ripple currents from DC/DCconverters 240, 242 and 244.

The +ve bus 264 and −ve bus 266 are connected as inputs to AC/DCconverter or inverter 256. AC/DC converter or inverter 256 takes the twoinputs with a DC voltage difference of +2ve and creates a three phaseoutput made up of phase A 258, phase B 260 and phase C 262. Each of thethree phases is an AC signal with a 120 degree phase offset to eachother phase and an RMS voltage with respect to neutral line 232 that isappropriate for the power system application.

The fuel cell power conversion system architecture 200 presentsadvantages in terms of modular design of the fuel system architecture.The architecture has adopted the dual bus architecture of the system ofFIG. 1, with a neutral line derived from the middle node of the two fuelcell segment components. This architecture is multiplied by three inparallel for the embodiment of FIG. 2. That is, there are six parallelbranches for DC/DC conversion, each containing a fuel cell segment pairin series with a neutral derived from the middle point of the pair.Thus, the fuel cell power conversion system of the embodiment of FIG. 2are designed using integral multiples of six fuel cell segments perAC/DC converter or inverter. An integral multiple of 3 fuel cell segmentpairs will make the power distribution along each phase easier and withminimum number of DC/DC converters. FIG. 3 shows a fuel cell powerconversion system architecture 300 with output transformers for afour-wire (with neutral) system. The architecture shows a fuel cellpower conversion system implementation with a single DC bus approach.

The system 300 comprises three parallel circuit branches 302, 304 and306, each having a fuel cell segment pair 308 and 310, 312 and 314, or316 and 318, respectively. Each fuel cell segment pair is connected inseries so that their respective voltage increases are summed.

The outputs of fuel cell segments 308, 312 and 316 are connected tobusses 320, 322 and 324 respectively, which lead to the inputs of DC/DCconverters 326, 328 and 330, respectively. The negative terminal of theseries connected fuel cell segments 308, 312 and 316 are connected toreference busses 332, 334 and 336 respectively, which are each connectedto a reference potential such as ground. It is clear that a referencepotential can be any relatively steady or convenient potential relativeto non-reference lines.

The outputs of each DC/DC converter 326, 328 and 330 are each connectedto +ve busses 338, 340 and 342 respectively. The +ve busses 338, 340 or342 are connected to a capacitors 344, 346 and 348, respectively, whichare in turn connected to reference busses 332, 334 and 336 respectively,to produce a voltage drop of +ve over each capacitor. The capacitors344, 346 and 348 also serve to smooth ripple currents produced by DC/DCconverters 326, 328 and 330 respectively.

The +ve busses 338, 340 and 342 and reference busses 332, 334 and 336serve as inputs to DC/AC converters or inverters 350, 352 and 354respectively. In contrast to the embodiments of FIGS. 1 and 2, eachphase of the three-phase current output has its own DC/AC converter orinverter. The output of each DC/AC converter 350, 352 and 354 produces arespective phase signal 356, 358 or 360 respectively and a neutral line362, 364 or 366 respectively. The use of three separate input branchesfor DC/AC inverter is particularly advantageous for the development ofthree-phase AC current, as each phase has its own neutral, positive andnegative busses, that is, each of the three phases runs from its own DCbus.

The three phase outputs 356, 358 or 360 are transformed to anapplication RMS voltage by transformers 368, 370 and 372 respectively.The neutral line outputs from the transformers are connected at node 382to generate a single neutral line.

The Figures also show in exemplary fashion methods that may be used forpower generation and conditioning. For example, FIG. 1 demonstrates amethod that may be used generally to convert the output of a directcurrent source to alternating current. As a first step, a first output106 of a first series connection of two fuel cell segments (segments 102and 104) is accepted at an input of a first DC/DC converter 110. Asecond output 108 of a first series connection of two fuel cell segments(segments 102 and 104) is accepted at an input of a second DC/DCconverter 112. A first output 114 of the first DC/DC converter 110 isaccepted at a first input 114 of a first inverter (i.e., DC/ACconverter) 124. A first output 116 of the second DC/DC converter 112 isaccepted at a second input 116 of the first inverter 124. A first ACoutput 126 is generated from the first and second inputs 114 and 116 ofthe first inverter 124 relative to a neutral line 122 connected to areference potential.

Such a method may be expanded as shown in FIG. 2, wherein three fuelcell segment pairs are shown (including segments 202 and 204; 206 and208; and 210 and 212 respectively), the outputs of each of which areaccepted at the inputs of two DC/DC converters (234 and 244; 236 and 242and 238 and 240 respectively) per fuel cell segment pair. The outputs ofthe DC/DC converters are electrically connected. The outputs of upperDC/DC converters 234, 236 and 238 are accepted at a first input ofinverter 256, while the outputs of lower DC/DC converters 240, 242 and244 are accepted at a second input of inverter 256. From these inputs,inverter 256 generates three phase outputs 258, 260 and 262 relative toneutral line 232.

Such a method may be also be expanded as shown in FIG. 3 to encompassmultiple fuel cell stack segment pairs with corresponding DC/DCconverters using a single inverter. Additionally, such methods may beused in the system of FIG. 3, wherein the first outputs 320, 322 and 324respectively of multiple fuel cell stack segments (fuel cells 308 and310; 312 and 314; and 316 and 318 respectively) are accepted at theinputs of DC/DC converters 326, 328 and 330 respectively. The outputs ofthe DC/DC converters and the second outputs of the fuel cell stacksegments are in turn accepted at respective inverters (DC/AC converters)350, 352 and 354. The inverters 350, 352 and 354 produce three phases356, 358 and 360.

The foregoing embodiments of the invention are intended to beillustrative in nature and not limiting. It will be clear to a person ofskill in the art that various modifications may be made without partingfrom the spirit and scope of the present invention, which should bedefined only by the following claims.

What is claimed is:
 1. A fuel cell circuit, comprising: a first fuelcell segment having positive and negative terminals; a second fuel cellsegment having positive and negative terminals; a neutral line; andwherein the negative terminal of the first fuel cell segment iselectrically connected to both the positive terminal of the second fuelcell segment and the neutral line.
 2. The apparatus of claim 1, furthercomprising a first DC/DC converter electrically connected to thepositive terminal of the first fuel cell segment.
 3. The apparatus ofclaim 2, further comprising a second DC/DC converter electricallyconnected to the negative terminal of the second fuel cell segment. 4.The apparatus of claim 3, wherein an output of the first DC/DC converteris electrically connected to a first input of an inverter.
 5. Theapparatus of claim 4, wherein an output of the second DC/DC converter iselectrically connected to a second input of the inverter.
 6. Theapparatus of claim 2, wherein the first fuel cell segment comprises aplurality of fuel cell stacks individually wired to be connected inseries.
 7. The apparatus of claim 5, further comprising first and secondcapacitors, the first capacitor connected between the output of thefirst DC/DC converter and the neutral line; the second capacitorconnected between the output of the second DC/DC converter and theneutral line.
 8. A power conversion module, comprising: a plurality ofDC/DC converter branches, each branch comprising a DC/DC converter; anda plurality of series connections comprising two fuel cell segments;wherein each DC/DC converter branch is connected to at most one seriesconnection comprising two fuel cell segments; and wherein the totalnumber of DC/DC converter branches is an integer multiple of three. 9.The power conversion module of claim 8, wherein the total number ofDC/DC converter branches is an integer multiple of six.
 10. The powerconversion module of claim 8, wherein each DC/DC converter branchcomprises an output bus, and wherein each of the DC/DC converter branchoutput busses is electrically connected to another DC/DC converterbranch output bus.
 11. The power conversion module of claim 8, whereineach DC/DC converter branch comprises an output bus, and wherein each ofthe DC/DC converter branch output busses is electrically connected to aninverter input.
 12. The power conversion module of claim 10, whereineach DC/DC converter branch comprises an output bus, and wherein each ofthe DC/DC converter branch output busses is electrically connected to aninverter input.
 13. The power conversion module of claim 12, wherein theDC/DC converter branches are divided into groups of two, the groups oftwo being defined in that each DC/DC converter branch in the group isconnected to the same series connection of two fuel cell segments.
 14. Afuel cell power conversion system comprising a plurality of powerconversion modules of claim
 13. 15. The power conversion module of claim8, wherein one half of the fuel cell segments are located on a positiveside of a neutral line and a remaining one half of the fuel cellsegments are located on a negative side of the neutral line.
 16. Amethod for converting DC to AC, comprising: accepting a first output ofa first series connection comprising two fuel cell segments at an inputof a first DC/DC converter; accepting a second output of the firstseries connection comprising two fuel cell segments at an input of asecond DC/DC converter; accepting a first output of the first DC/DCconverter at a first input of a first inverter; accepting a first outputof the second DC/DC converter at a second input of the first inverter;and generating a first AC output from the first and second inputs of thefirst inverter relative to a neutral line connected to a referencepotential.
 17. The method of claim 16, further comprising: accepting afirst output of a second series connection comprising two fuel cellsegments at an input of a third DC/DC converter; accepting a firstoutput of the third DC/DC converter at a first input of a secondinverter; accepting a second output of the second series connection at asecond input of the second inverter; generating a second AC output fromthe first and second inputs of the second inverter relative to theneutral line; accepting a first output of a third series connectioncomprising two fuel cell segments at an input of a fourth DC/DCconverter; accepting a first output of the fourth DC/DC converter at afirst input of a third inverter; accepting a second output of the thirdseries connection at a second input of the third inverter; andgenerating a third AC output from the first and second inputs of thethird inverter relative to the neutral line.